Somebody screwed up. And it appears to have been a violation of FCC rules:

A source with knowledge of the situation says an honest mistake was made, but a mistake nonetheless, when a third-party vendor helping Monterey Park manage its alert system perhaps misunderstood instructions from the city and blasted the "this is only a test" message -- live!

In other words, it was not supposed to reach your phone.

Test alerts are not supposed to go live, or use live code, the source says. In fact, live tests that rock our mobile phones are not supposed to happen at all -- the FCC apparently forbids it.

For good reason: The messages shook up many Angelenos. The first went out shortly after 2:15 p.m. yesterday, but they apparently happened more than once afterward: